Chromium plating bath containing chromic compound and organic component

ABSTRACT

DEPOSITION OF BRIGHT DECORATIVE CHROMIUM PLATE FROM CHROMIUM PLATING MEDIA CONTAINING CHROMIC COMPOUND, IS ENHANCED IN THE LOW CURRENT DENSITY AREA, ESPECIALLY AFTER WORKING, BY THE PRESENCE IN THE MEDIUM OF MINOR AMOUNTS OF ORGANIC SUBSTANCE. THE MEDIUM CONTAINS TRIVALENT CHROMIC-COMPOUND FOR PLATING WHICH HAS AT LEAST CARBOXYLIC ACID CONSTITUENTS AND FURTHER EXHIBITS READY WATER SOLUBILITY. SUCH ORGANIC SUBSTANCE IS EXEMPLIFIED BY PHENOL, CRESOLS, XYLENOLS, RESORCINOL, UREA, MELAMINE, ACRYLAMIDE, AND ANILINE. TYPICALLY, ONLY VERY MINOR AMOUNTS OF SUCH SUBSTANCES NEED BY PRESENT IN THE MEDIUM.

United States Patent M 3 706,643 CHROMIUM PLATING BATH CONTAINING CHROMIC COMPOUND AND ORGANIC COMPONENT Francis Huba, Painesville, Ohio, assignor to E. I. du Pont de Nemours and Company, Wilmington, Del. No Drawing. Filed Feb. 19, 1971, Ser. No. 117,163

Int. Cl. C231) 5/06 US. Cl. 204-51 11 Claims ABSTRACT OF THE DISCLOSURE Deposition of bright decorative chromium plate from chromium plating media containing chromic compound, is enhanced in the -low current density area, especially after working, by the presence in the medium of minor amounts of organic substance. The medium contains trivalent chromic-compound for plating which has at least carboxylic acid constituents and further exhibits ready water solubility. Such organic substance is exemplified by phenol, cresols, xylenols, resorcinol, urea, melamine, acrylamide, and aniline. Typically, only very minor amounts of such substances need be present in the medium.

BACKGROUND OF THE INVENTION Decorative chromium plating from baths containing chromium in the trivalent state in association with at least carboxylic acid constituents has offered promise for commercial use; US. Pat. 3,006,823, for example, describes a recently developed aqueous electrolytic plating bath containing a chromium complex of chromic ion and carboxylic acid. However, in working such baths, desirable performance in chromium rate of deposition in the low current density area is not always achieved for an extended period. Undesirable plate may eventually be experienced in the low current density region, not only in plating thickness, but also with regard to the extent or range for which an extendible plate thickness can be obtained.

SUMMARY OF THE INVENTION It has now been found that in the plating of decorative chromium plate with baths containing chromium in the trivalent state in association with carboxylic acid constituents,'the presence of an organic component can enhance the deposition of the place in the low current density area during extended working of the plating medium. Such enhancement may be exhibited by an increase in thickness of the plating deposit and can be achieved even though such media fail to demonstrate any formation or build up of hexavalent chromium. For example, the analysis of representative plating baths by the diphenylcarbazide test has always been negative.

In part, the invention is directed to an aqueous electrolytic plating bath for the plating of bright chromium plate, which bath has enhanced electrodeposition of such plate, and comprises a complex, water-soluble chromic compound for the deposition of chromium plate, with the complex containing carboxylic acid constituents, and the bath containing an organic component. The organic component provides in the bath at least one compound selected from the group consisting of aromatic hydrocarbons having at least one hydroxyl group on the aromatic ring, and nitrogen-containing, organic compounds.

The invention is further directed to the method of chromium plating an article with a decorative chromium plate from a bath showing enhanced deposition in the low current density area and is also directed to the method of enhancing such electrodeposition of bright chromium plate from an aqueous chromium plating bath.

Patented Dec. 19, 1972 DESCRIPTION OF THE PREFERRED EMBODIMENTS The chromic compound contains carboxylic acid constituent, e.g., the chromic carboxylate of the aqueous electrolytic plating baths disclosed in US. Pat. 3,006,823. As taught therein, the chromic compound is preferably a chromic carboxylate of an alpha-hydroxy carboxylic acid, for example glycolic and lactic acids, although other acids can be employed such as will be discussed more particularly hereinbelow. The carboxylates may be added to the plating bath as such or, as taught in U.S. Pat. 3,021,267, they may be provided by dissolving chromic hydroxide or carbonate or even metallic chromium in the carboxylic acid and the pH adjusted with sodium hydroxide or carbonate. As used herein the term chromic carboxylate is meant to refer to such compounds of trivalent chromium and carboxylic acid as are exemplified by such patents.

However, the bath may also contain some to all of a water-soluble chromic compound containing carboxylic acid constituents plus halogen constituents which can be chlorine, fluorine, bromine, iodine, or mixtures thereof. However, in typical commercial plating operation, bromine and iodine are often not used, for economy and to avoid evolution of visible noxious fumes at the anode. Therefore, chlorine and fluorine are almost exclusively used.

Although it is not meant that the invention be limited to chromium plating from liquid medium containing a chromic compound having an acid constituent representative of only especial groups of carboxylic acids, such acids which can or have been used for the chromic compounds are typically exemplified by dicarboxylic and monocarboxylic acids, free from carbon-to-carbon unsaturation, and with or wit-bout hydroxyl groups. For plating efiiciency and water solubility, advantageously these acids are non-aromatic acids containing less than about 10 carbon atoms; representative acids include glycolic acid, lactic acid, formic acid, oxalic acid, and their mixtures. Preferably, for enhanced plating performance, plus economy, although it is not meant that this invention be bound to bath-s wherein the chromic compound contains only carboxylic acid constituents alone, the baths used virtually always contain a chromic compound which has carboxylic acid constituents supplied at least in part by glycolic acid. A compound of any of these acids such as a salt or an ester thereof, which acts in any of the reactions, such as those discussed in more detail hereinbelow whereby the complex is formed, in the same manner as the free acid, can be used.

In preparing a plating bath containing a chromic carboxylate, typically chromic acid can be reacted with the carboxylic acid. For example, to reduce one mole of the chromic acid, 0.5 mole of glycolic acid may be used and 1.0 mole of glycolic acid is generally present to form the desired complex. Typically excess acid, e.g., a 0.1 mole excess, is present to insure reduction and complex formation. Thus, more particularly, chromic acid present as a solution in water, may be slowly added to glycolic acid, also dissolved in water, at -100 C. When addition is complete, the solution can be heated to reflux and typically maintained at reflux for about one hour or more to complete reaction.

When halogen is incorporated in the complex, the complex can be prepared by any of several methods. One method is the straightforward combination of chromium metal with carboxylic acid plus hydrochloric acid. When such combination includes particulate chromium metal to reduce reaction time, the reaction can be highly exothermic, and therefore caution needs be taken in carrying out same. Typically for enhanced reaction efliciency, as the reaction proceeds and the evolved heat starts to diminish, external heating is applied; and, where the reaction proceeds in aqueous medium such external heating can involve refluxing of the reaction mixture to augment completion of the reaction.

The complex of this type may also be prepared from the carboxylic acid and hydrochloric acid in admixture with chromic acid, typically charged to the reaction medium as a solution of chromic acid in water. The chromic acid can be supplied by any of the suitable substances for forming chromic acid in water, e.g., chromium trioxide. The reaction resulting from this method is also exothermic and caution in the use of such method is also thus advisable. These complexes may further be prepared by reaction of chromic halide, with such halide corresponding to the halide that is to be present in the complex; the chromic halide is reacted with the carboxylic acid, with this reaction further involving the addition of strong base, e.g., an alkali metal hydroxide. For example, CrF '9H O may be used in this method and will readily yield a chromium/carboxylic acid/fluoride complex involving exothermic reaction conditions.

These carboxyl containing complexes virtually always contain a molar ratio of chromium atoms to carboxylate constituent within the range of 1:0.7 to 1:3.0. Where halogen is present the complex essentially always has a molar ratio of chromium atoms to halogen atoms within the range of 120.1 to 1:3.5. Especially preferred ratios, based upon desirable plating performance and economy, can depend upon the acid and also upon the halogen constituent when such is present. Thus for example, for a chromic carboxylate prepared with glycolic acid, the ratio of the chromic ion to glycolic is preferably maintained within the range from about 1:1.1 to 1:2.1. For a complex containing a substantial amount of the glycolic acid for the carboxylate, which complex further contains chloride as the major amount, to. all,of1the halogen, the ratio of chromium atoms to halogen is preferably within the range of about 120.4 to 1:1. However, when the halogen in such a complex is preponderantly, to all, fluoride, the ratio of chromium atoms to halogen is preferably within the range of 1:2.6 to 1:3.2.

The complex is generally present in the bath in an amount to provide from about 25 to about 150 grams of chromium per liter, that is, the molar concentration of chromium in the plating medium is generally within the range from about 0.5 to about 3.0, although for the baths containing chromic carboxylate as little as 0.1 mole of chromium in the plating medium is serviceable. The more highly concentrated baths having augmented viscosity are not well suited for deposition of chromium onto a substrate immersed therein. Thus such baths having molar concentration of chromium above about 1.5 are typically used in portable plating devices for'spot plating, e.g., brush plating. Regardless of plating method, the bath is one where the complex is present in a liquid medium supplied all, or virtually all, by water. For example, in a bath containing a chromic carboxylate where the carboxylic acid is present in excess and the acid is liquid at normal temperature and pressure, a minor contribution to the medium can come from the excess acid.

The organic component of the bath is obtained by providing the bath with at least one compound of an aromatic hydrocarbon having at least one hydroxyl group on the aromatic ring, or a nitrogen-containing organic compound. Such substances are generally supplied to the medium in an amount from about 0.1 weight percent, to not substantially in excess of about 5 weight percent, basis weight of the liquid in the plating medium, for economy plus efllcient enhancement of low current density plating. Preferably, for ease of blending with the plating medium, the one or more substances used for the organic component are readily water dispersible, e.g., are water soluble in an amount of at least about one gram per liter, basis liters of the plating medium:

Such substances as may or have been used for the organic component include the hydroxyl-containing, aromatic hydrocarbons phenol, resorcinol, the cresols, the xylenols, p-tert-butylphenol, p-phenylphenol, and their mixtures. The nitrogen-containing organic compounds include urea, melamine, thiourea, aniline, dicyanodiamide, toluenesulfonamide, benzoguanamine, ethyleneurea, acrylamide, and their mixtures. Some of these compounds can be thought of in polymer terminology as poly-(alkylene amide)precursors, and thus, for convenience, this term may be used hereinafter to refer to all of these compounds collectively. For addition to the bath, the organic component substances can be added in any convenient manner, for example as a water solution or as a blend in solution with chromic compound; such a blend is then added to the bath in part for replenishing the bath during working. The organic component is generally not present in the bath when the bath is freshly prepared although such component may be present and if so, is usually added before the freshly prepared bath is electrolyzed. In general the bath need not contain any organic component until working of the bath is accompanied by some drop oflf in plating speed in the low current density area.

Although particular baths may contain certain specified, additional substances and in particular amounts, e.g., the amounts of free carboxylic acids specified for the bath of US. Pat. 3,021,267, in general the bath can also contain a salt of a strong acid preferably, for economy, an alkali metal salt. Such salts enhance the conductivity achieved in the electroplating operation. Most preferably, for economy, the cation of the salt is sodium, potassium or their mixtures, and the strong acid anions should typically be those of an acid having a dissociation constant of at least K=l0" for example, chloride. The plating bath usually contains between about 50-200 grams per liter of such salts. The bath can also contain boric acid, or an equivalent to boric acid in aqueous solution, such as borax, boron oxide, or sodium oxyfluoborate. Such compounds operate in the bath to augment the rate of deposition of the chromium and are typically used in an amount between about 10-70 grams per liter of bath.

Before deposition of chromium, the bath pH is adjusted to within a range depending upon the complex present, e.g., for a bath containing a major amount of one simple chromic carboxylate complex, the bath is adjusted to a pH within the range from about 1.5-3.0. For a bath wherein the major amount of complex is contributed by such complex further containing halogen atoms, which complex for overall plating efliciency and for an augmented extended bright range is preferred, the bath is preferably adjusted to a pH within the range from about 2.0-3.5. Therefore, for most of the plating baths in the present invention, such baths will be at pH between 1.5- 3.5. However, there can be achieved an acceptable bright chromium plate for such halogen containing complexes, and enhanced deposition from the present invention at the low current density area, when the bath is maintained within the range from about 1.8-4.9. Also, for complex chromic carboxylate baths having blends of chromic carboxylates, as shown in US. Pat. 3,021,267, the bath can be maintained within a pH of between 2.7 and 4.5. Thus, for these particular complexes, such acceptable ranges are used.

For such baths having a halogen containing complex, the most desirable pH range can depend upon the particular make-up of the complex present in the plating medium with, for example, a complex containing a substantial amount of fluorine as the halogen being preferably maintained at a slightly more elevated bath pH than a bath where chlorine supplies the major amount of a halogen. Any adjustment of the bath pH can be readily carried out with a base, particularly alkali metal carbonates or hydroxide, with sodium or potassium hydroxide or their mixtures being preferred. Before addition to the bath, such material for adjusting the bath pH can be initially dissolved in water and the water solution then added to the bath.

The temperature of the bath during plating, for efficiency and economy is in part dependent upon the complex present in the bath, with such baths as contain a major amount of chromic carboxylate, being typically maintained during plating at a temperature within the range from room temperature up to about 90 C., but more typically up to about 65 C. For those baths where the major amount of the complex contains a halogen, the temperature is advantageously for enhanced plating performance not substantially above about 50 C.

During plating, the object to be plated is made the cathode, for example, immersed in the plating bath, or the cathode in a spot plating system using a portable plating device supplying the electrolyte and a positive source of electrical current, e.g., brush plating operation, where the plating medium is contained in the brush, and an inert anode is used such as a carbon, graphite, platinum, or platinized titanium anode. Exemplary cathode substrates for receiving the plate include metal such as steel, brass, copper, copper alloys, bronze, zinc die castings, and nickel. Additionally such plating can be performed on plastic surfaces which are activated or prepared for an electroplating operation.

The plating can be typically carried out in any vessel useful for chromium electroplating such as tanks lined with corrosion resistant material including glass, ceramic material, polyvinyl chloride and the like. Also, electrodeposition with such plating baths containing their organic additives can be performed by any conventional plating technique including rotating receptacle coating apparatus immersed in the plating bath. Although diaphragm compartment cells may be employed for plating, they are not preferred for economy.

The following example shows a way in which the invention has been practiced but should not be construed as limiting the invention. Unless otherwise specified, plating tests in the example are conducted in a modified Hull cell. The standard Hull cell is a trapezoidal box of nonconductive material at the opposite ends of which are positioned anode and cathode plates, as has been more particularly described in US. Pat. No. 2,149,344. For either the standard or the modified Hull cell, it is possible to easily determine the effective plating range of a plating composition under varying conditions. The current density at any point on a cathode is determined according to the formula A=C(27.748.7 log L) wherein A is the currentdensity at the selected point, C is the total current applied to the cell and L is the distance of the selected point from the high current density end of the plate.

In the modified version of the Hull cell used herein, /2-inch holes are introduced in the parallel sides of the cell adjacent the anode and cathode whereby, upon immersion of the cell in another vessel containing plating solution, into which vessel the cell will fit very closely, and also improved electrolyte circulation and consequent improved temperature control is afforded, as more particularly described in an article appearing in Plating, vol. 46, No.3 (1959), page 257.

EXAMPLE Into a container there is placed 0.78 mole of chromium metal, 1.8 mole of glycolic acid of 70 percent strength, that is, 70 percent of glycolic acid and a balance of water, and 0.4 mole of 37.3 percent strength of hydrochloric acid which is 37.3 percent by weight HCl in water. The container is covered and good ventilation is provided. After the ingredients are placed together in the container, dissolution of the chromium sarts slowly but gradually increases thus supplying heat to the reaction period. As the reaction continues the temperature of the reaction medium reaches 77 C. without external heating and the chromium metal can be seen by visual inspection to be substantially dissolved. As the temperature starts to subside from 77 C., external heating is applied and the temperature of the reaction medium is permitted to reach C. after al the chromium is dissolved. Total reaction time, i.e., to complete chromium metal dissolution, is about 6 hours. Thereupon the solution is heated at reflux, reaching a temperature of 105 C., for about 2 hours, and is thereafter permitted to cool.

The resulting complex has a molar ratio of chromium to glycolic acid of 1:231 and of chromium to chloride of 1:0.513. To prepare a bath for plating the following substances are added to obtain the following concentrations: 55 grams per liter (g./l.) of boric acid, g./l. of potassium chloride and sufficient 40 percent strength sodium hydroxide, that is, 40 weight percent NaOH and a balance of water, to provide a pH of the bath of about 3. A 1500 milliliter portion of this bath, supplying 40 g./l. of chromium, is taken for plating tests and before plating is combined with 0.99 percent by volume of an about 40 percent solution of formaldehyde in water for accelerated ageing of the bath.

The bath is electrolyzed at a rate of about 20-30 amphours per gallon and is thereby prepared for plating in the above-described modified Hull cell. In the Hull cell, graphite rod anodes are used and the cathode employed is 3 by 2%" brass panels, one panel for each test, each panel being nickel coated prior to use in the cell. Each test is carried out using 10 amperes current for a 3-minute cycle. I

The results shown in the table below give the rates of deposition for two specific current density levels. The chromium thickness reported at each density level is in micro-inches per 3-minute cycle. The current density range shown in the table is in amperes per square foot (ASF).

The above results demonstrate the excellent restoration of the plating performance, e.g., in the low current density range, that can be achieved by addition of a very minor amount of the hydroxyl-containing, aromatic hydrocarbon.

What is claimed is:

1. An aqueous electrolytic plating medium for the plating of bright chromium plate, and having enhanced electrodeposition of said plate, which comprises:

(A) a complex, water-soluble chromic compound for the. deposition of chromium plate and containing non-aromatic carboxylic acid constituent containing less than about 10 carbon atoms and selected from the group consisting of dicarboxylic acids, monocarboxylic acids, monocarboxylic and dicarboxylic acids containing at least one hydroxyl group, and mixtures thereof; and,

(B) an organic component having ready water dispersibility and supplying substance in an amount not substantially above about 5 weight percent, basis weight of said medium, selected from the group consisting of aromatic hydrocarbon having at least one hydroxyl group on the aromatic ring, and nitrogencontaining, poly(alkylene amide) precursors.

2. The plating medium of claim 1 wherein said medium has a molar concentration of chromium of at least about 0.1.

3. The plating medium of claim 1 wherein said aromatic hydrocarbons are selected from the group consisting of phenol, resorcinol, cresols, xylenols, p-tert-butylphenol, p-phenylphenol, and their mixtures.

4. The plating medium of claim 1 wherein said medium is maintained at a temperature not substantially above about 90 C., and articles to be plated are immersed therein with the complex being present in said medium in an amount supplying not substantially above about 75 grams of chromium per liter of the medium.

5. The plating medium of claim 1 wherein said nitrogencontaining poly(alkylene amide)precursors are selected from the group consisting of urea, melamine, thiourea, aniline, dicyanodiamide, toluenesulfonamide, benzoguanamine, ethyleneurea, acrylamide, and their mixtures.

6. The plating medium of claim 1 wherein said medium additionally contains a salt of a strong acid having a dissociation constant of at least K= land a substance selected from the group consisting of boric acid, a substance supplying boric acid equivalent in aqueous solution, and mixtures thereof.

7. The plating medium of claim 6 wherein the cation of said salt is selected from the group consisting of sodium, potassium, and their mixtures, and the anion of said salt is selected from the group consisting of perchlorate, chloride, and mixtures thereof, with said medium containing between about 50-200 grams per liter of said salts.

8. The plating medium of claim 6 wherein said substance supplying boric acid equivalent in aqueous solution is selected from the group consisting of borax, boron oxide, sodium oxyfiuoborate, and mixtures thereof and said medium contains between about 10-70 grams per liter of said substance.

9. The plating medium of claim 1 wherein said chromic compound additionally contains halogen constituents selected from the group consisting of chloride, fluoride, mixtures thereof and mixtures thereof with other halides, and carboxylic acid constituents supplied by acids selected from the group consisting of glycolate, lactate, oxalate and mixtures thereof, said compound having a molar ratio of chromium atoms to carboxyl constituent within the range of 1:07 to 1:3, and a molar ratio of chromium atoms to halogen atoms within the range of 1:0.1 to 1:35.

10. A composition for sustaining plating from an electroplytic plating medium, while enhancing the electrodeposition of chromium plate from said medium, which composition comprises a blend of:

(A) a complex, water-soluble chromic compound for the deposition of chromium plate and containing nonaromatic carboxylic acid constituent containing less than about 10 carbon atoms and selected from the group consisting of dicarboxylic acids, monocarboxylic acids, monocarboxylic and dicarboxylic acids containing at least one hydroxyl group, and mixtures thereof; and,

(B) an organic component having ready water dispersibility and supplying substance in an amount not substantially above about 5 weight percent, basis weight of said medium, selected from the group consisting of aromatic hydrocarbon having at least one hydroxyl group on the aromatic ring, and nitrogencontaining poly(alkylene amide) precursors.

11. A method of enhancing electrodeposition of bright chromium plate at low current density from an aqueous chromium plating medium containing a complex, watersoluble chromic compound for the deposition of chromium plate and containing carboxylic acid constituent, which method comprises:

(1) blending with said liquid medium an organic component having ready water dispersibility and supplying substance in an amount not substantially above about 5 Weight percent, basis weight of said medium, selected from the group consisting of aromatic hydrocarbon having at least one hydroxyl group on the aromatic ring, and nitrogen-containing, poly (alkylene amide)precursors.

(2) passing a current at low current density between an anode and a cathode in contact with said liquid medium.

References Cited UNITED STATES PATENTS FOREIGN PATENTS 3/1969 Great Britain 20451 F. C. EDMUNDSON, Primary Examiner 

